Optical ring network architecture

ABSTRACT

An optical ring network architecture including a number (N) of multi-add/drop filters, such as filters formed using pairs of frequency routers. Each multi-add/drop filter is coupled to two other multi-add/drop filters using N−2 transmission media, such as optical fibers, to form a ring. The network includes a number (N) of terminal stations associated with the multi-add/drop filters. A terminal station (p) is coupled with, and receives information from, its associated multi-add/drop filter (p) through a single optical fiber. The terminal station p is coupled with, and transmits information in a first direction around the ring to, a multi-add/drop filter p+1 through a single optical fiber. Communications from terminal station p to each other terminal station in the first direction are assigned one of N−1 wavelengths where no two wavelengths on a given optical fiber are associated with communications between terminal stations in the same direction.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/756,025, filed Apr. 7, 2010, which is currently allowed and is acontinuation of U.S. patent application Ser. No. 12/173,679, filed Jul.15, 2008, (now U.S. Pat. No. 7,764,884) and is a continuation of U.S.patent application Ser. No. 11/539,772, filed Oct. 9, 2006, (now U.S.Pat. No. 7,412,171) which is a continuation of U.S. patent applicationSer. No. 10/324,344, filed Dec. 20, 2002, (now U.S. Pat. No. 7,123,837)which is a continuation of U.S. patent application Ser. No. 09/175,171,filed Oct. 20, 1998, (now U.S. Pat. No. 6,567,197), all of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to optical networks. More particularly,the present invention relates to an optical ring network architecture.

BACKGROUND OF THE INVENTION

The use of optical networks can dramatically increase the amount ofinformation, such as telephone, video and Internet information, that canbe communicated between network users as compared to traditionalnetworks. Such an optical network can, for example, connect a number ofterminal stations through a number of parallel optical fibers. When auser at a first terminal station wants to transmit information to a userat a second terminal station, the information is transmitted through oneof the optical fibers with a dedicated wavelength of light.

The user at the first terminal station may also want to simultaneouslytransmit information to a number of different users located at a numberof different terminal stations. Moreover, users at a number of differentterminal stations may want to transmit information to each othersimultaneously. Creating a network that lets all users communicate withall other users simultaneously, however, tends to increase the number ofoptical fibers that must be used in the network. Unfortunately, eachadditional optical fiber that is used can be very expensive to installand maintain. In addition, some networks need to be fully “restorable,”meaning that each user can still communicate with each other user whenany one of the optical fibers fail. This also tends to increase thenumber of optical fibers required in the network.

One way to reduce the number of optical fibers in a network is to useWavelength Division Multiplexing (WDM). In a WDM network, a set ofwavelengths, such as λ₁, λ₂ . . . λ_(n), are used so that severalcommunications can be simultaneously transmitted over a single opticalfiber using different wavelengths. To increase the amount of informationthat can be transmitted over the network, and to reduce the cost ofoptical transmitters, receivers and routers, it is desirable to keep thenumber of different wavelengths used in the network as small aspossible.

In addition, to avoid interference in the network a single wavelengthshould not be used to simultaneously transmit different information overthe same optical fiber in the same direction. Moreover, it may benecessary to amplify one or more signals being transmitted over anoptical fiber in the network. In this case, it is desirable thatinformation is not simultaneously transmitted over the same opticalfiber using the same wavelength, even if the transmissions are inopposite directions.

In view of the foregoing, it can be appreciated that a substantial needexists for an optical network architecture that reduces the number ofoptical fibers and wavelengths used in the network and solves the otherproblems discussed above.

SUMMARY OF THE INVENTION

The disadvantages of the art are alleviated to a great extent by anoptical ring network architecture including a number (N) ofmulti-add/drop filters, such as filters formed using symmetrical pairsof frequency routers. Each multi-add/drop filter is coupled to two othermulti-add/drop filters using N−2 transmission media, such as opticalfibers, to form a ring. The network also includes a number (N) ofterminal stations associated with the multi-add/drop filters. A terminalstation (p) is coupled with, and receives information from, itsassociated multi-add/drop filter (p) through a single optical fiber. Inaddition, the terminal station p is coupled with, and transmitsinformation in a first direction around the ring to, a multi-add/dropfilter p+1 through a single optical fiber.

Communications from terminal station p to each other terminal station inthe first direction are assigned one of N−1 wavelengths such that no twowavelengths on a given optical fiber are associated with communicationsbetween terminal stations in the same direction. When there are fourterminal stations, for example, the second terminal station maycommunicate with the first, third and fourth terminal stations usingwavelengths λ₁, λ₃ and λ₂, respectively. Moreover, all wavelengths on agiven optical fiber may be associated with a communication betweenterminal stations in either the first or second direction. As a result,each terminal station can communicate with each other terminal stationsimultaneously using wavelength division multiplexing and N−1wavelengths. The network may also be bi-directional such that eachterminal station p is coupled with, and transmits information in asecond direction opposite the first direction to, a multi-add/dropfilter p−1 through a single optical fiber.

With these and other advantages and features of the invention that willbecome hereinafter apparent, the nature of the invention may be moreclearly understood by reference to the following detailed description ofthe invention, the appended claims and to the several drawings attachedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical ring network architectureconnecting four terminal stations according to an embodiment of thepresent invention.

FIG. 2 is a bi-directional optical ring network architecture accordingto another embodiment of the present invention.

FIG. 3 shows the optical ring network architecture of FIG. 1 in greaterdetail according to an embodiment of the present invention.

FIG. 4 illustrates transmissions from the first terminal station of theoptical ring network shown in FIG. 3 according to an embodiment of thepresent invention.

FIG. 5 illustrates transmissions from the second terminal station of theoptical ring network shown in FIG. 3 according to an embodiment of thepresent invention.

FIG. 6 illustrates transmissions from the third terminal station of theoptical ring network shown in FIG. 3 according to an embodiment of thepresent invention.

FIG. 7 illustrates transmissions from the fourth terminal station of theoptical ring network shown in FIG. 3 according to an embodiment of thepresent invention.

FIG. 8 shows how the architecture of FIG. 3 may be used in abi-directional optical ring network according to another embodiment ofthe present invention.

DETAILED DESCRIPTION

The present invention is directed to an optical ring networkarchitecture. Referring now in detail to the drawings wherein like partsare designated by like reference numerals throughout, there isillustrated in FIG. 1 a block diagram of an optical ring networkarchitecture connecting four terminal stations 10, 20, 30, 40 accordingto an embodiment of the present invention. The network includes a number(N) of multi-add/drop filters 15, 25, 35, 45, and each multi-add/dropfilter is coupled to two other multi-add/drop filters to form a ring.The operation of the multi-add/drop filters is described in detail withrespect to FIGS. 3 to 8.

The network also includes a number (N) of terminal stations, and eachterminal station is associated with a different multi-add/drop filter.Although the network shown in FIG. 1 illustrates a network connectingfour multi-add/drop filters and four terminal stations, other numbers ofmulti-add/drop filters and terminal stations may be used instead.

According to an embodiment of the present invention, each terminalstation is capable of communicating with each other terminal stationsimultaneously using wavelength division multiplexing and N−1wavelengths, such as λ₁, λ₂ and λ₃, as follows. The multi-add/dropfilters are coupled to each other using N−2 transmission media, such asoptical fibers in an optical fiber trunk. Each terminal station (p) iscoupled with, and receives information from, its associatedmulti-add/drop filter (p) through a single transmission medium. Forexample, the second station 20 receives information from the networkthrough a single transmission medium connected to multi-add/drop filter25. Note that in the network shown in FIG. 1, terminal station “p” maybe any of the four terminal stations 10, 20, 30, 40 illustrated.

In addition, each terminal station p is coupled with, and transmitsinformation in a first direction around the ring to, a multi-add/dropfilter p+1 through a single transmission medium, wherein multi-add/dropfilter p+1 is the multi-add/drop filter neighboring multi-add/dropfilter p in the first direction. For example, the second station 20sends information to the network through a single transmission mediumconnected to the multi-add/drop filter 35 associated with the thirdstation 30. Because the network is arranged in a ring, the fourthstation 40 sends information to the network through a singletransmission medium connected to the multi-add/drop filter 15 associatedwith the first station 10.

Communications from the terminal station p to each other terminalstation in the first direction are assigned a different one of the N−1wavelengths. For example, the second station 20 may send information tothe third station 30, in the direction from left to right in FIG. 1,using λ₁ (not labeled in FIG. 1). Similarly, the second station 20 maysend information in the first direction to the fourth station 40 usingλ₃ and to the first station 10 using λ₂. A detailed illustration of anetwork in which communications from a terminal station to each otherterminal station in the first direction are assigned a different one ofthe N−1 wavelengths is discussed with respect to FIGS. 4 to 7.

FIG. 2 is a bi-directional optical ring network architecture accordingto another embodiment of the present invention. An additionaltransmission medium, shown as a solid arrow in FIG. 2, is used to coupleeach terminal station p with a multi-add/drop filter p−1, whereinmulti-add/drop filter p−1 is the multi-add/drop filter neighboringmulti-add/drop filter p in a second direction opposite the firstdirection. The additional transmission medium is used to transmitinformation in this second direction, or from right to left in FIG. 2.In addition, wavelengths may be selected, as explained with respect toFIG. 8, such that no two of the N−1 wavelengths, such as λ₁, λ₂ and λ₃,on a given transmission medium are associated with communicationsbetween terminal stations in the same direction. Moreover, all of theN−1 wavelengths on a given transmission medium may be associated withcommunications between terminal stations in either the first or seconddirections.

In this way, the optical ring network may be fully restorable in theevent that a single transmission medium fails. That is, if an opticalfiber breaks such that a terminal station can no longer transmit to oneor more remaining terminal stations in the first direction, the terminalstation can still communicate with those remaining terminal stations inthe second direction. Moreover, the capability of the network can bedoubled when there is no failure by sending information in bothdirections around the ring.

FIG. 3 shows the optical ring network architecture of FIG. 1 in greaterdetail according to an embodiment of the present invention. Eachterminal station includes a Multiple Transmitter (MT) configured totransmit information to the network and a Multiple Receiver (MR)configured to receive information from the network. For example, thesecond station includes a multiple receiver 210 and a multipletransmitter 220. As used herein, a “multiple” transmitter or receivermay be, for example, a device with an array of transmitters orreceivers. This lets the device transmit or receive information usingmultiple wavelengths simultaneously.

Each of the multi-add/drop filters comprises a symmetrical pair offrequency routers. For example, the multi-add/drop filter associatedwith the second station comprises an “input” frequency router 530 and an“output” frequency router 540. Each of the frequency routers has 3 inputports, located on the left in FIG. 3, and 3 output ports, located on theright in FIG. 3. Note that only two of the three output ports are used.Although routers and ports are referred to herein as being either“input” or “output” devices, it should be noted that signals may alsopass through the routers and ports in the opposite direction.

A detailed explanation of frequency router and multi-add/drop filtertechnology is provided in U.S. Pat. No. 5,002,350 to Dragone and U.S.Pat. No. 5,367,586 to Glance et al., the entire disclosures of which ishereby incorporated by reference. The operation of the input and outputfrequency routers shown in FIG. 3 will now be briefly described.

As shown in FIG. 3, each input frequency router 510, 530, 550, 570 hasthree input ports and three output ports. When an optical beam comprisedof wavelengths λ₁, to λ₃ enters one of the input frequency routers 510,530, 550, 570 at input port 1, λ₁ exits at output port 1, λ₂ exits atoutput port 3, and λ₃ exits at output port 2. In general, as shown inTable I, when λ₁ enters input port X, λ₁ exits from output port X. Whenλ₂ enters input port X, λ₂ exits from output port X+2, and when λ₃enters input port X, λ₃ exits from output port X+1. Moreover, the inputfrequency routers 510, 530, 550, 570 have a “cyclical routing quality”in that when the solution of this general equation results in an outputport greater than 3, the wavelength “wraps” around to the top of thefrequency router. For example, when λ₂ enters input port 3, λ₂ exitsfrom output port 2.

TABLE I Input Frequency Router Connection Table Output Output OutputPort 1 Port 2 Port 3 Input Port 1 λ₁ λ₃ λ₂ Input Port 2 λ₂ λ₁ λ₃ InputPort 3 λ₃ λ₂ λ₁

When an optical beam comprised of wavelengths λ₁ to λ₃ enters one of theoutput frequency routers 520, 540, 560, 580 at input port 1, λ₁ exits atoutput port 1, λ₂ exits at output port 2, and λ₃ exits at output port 3.In general, as shown in Table II, when λ_(x) enters input port X, λ_(x)exits from output port (X+x−1). As with the input frequency routers 510,530, 550, 570, the output frequency routers 520, 540, 560, 580 also havea cyclical routing quality.

TABLE II Output Frequency Router Connection Table Output Output OutputPort 1 Port 2 Port 3 Input Port 1 λ₁ λ₂ λ₃ Input Port 2 λ₃ λ₁ λ₂ InputPort 3 λ₂ λ₃ λ₁

Finally, both the input and output frequency routers have the propertyof “reciprocity,” meaning that when a signal enters an output port, i.e.travels right to left in FIG. 3, it exits from the same input portassociated with travel in the other direction. For example, Table IIdemonstrates that when λ₃ enters input port 2 it exits from output port1. Thus, if λ₃ enters output port 1, reciprocity requires that it exitfrom input port 2. Note that input and output frequency routers havingthree input and output ports are used to illustrate an embodiment of thepresent invention, input and output frequency routers having a differentnumber of input and output ports may be used instead, such as when morethan four terminal stations are present in an optical ring network.

Referring again to FIG. 3, the multi-add/drop filter associated with thesecond station, comprised of input frequency router 530 and outputfrequency router 540, will now be described in detail. Output ports 1and 2 of the input frequency router 530 are coupled to input ports 1 and2 of the output frequency router 540, respectively, and output port 3 ofthe input frequency router 530 is coupled to the second station'smultiple receiver 210. Output ports 2 and 3 of the output frequencyrouter 540 are respectively coupled to input ports 1 and 2 of theneighboring input frequency router 550, associated with the thirdstation. Finally, the second station's multiple transmitter 220 iscoupled to input port 3 of the neighboring input frequency router 550.The other multi-add/drop filters are similarly constructed.

Note that input port 3 and output port 1 of the output frequency router540 are not used. These ports are used with respect to communicationsthrough the network in the opposite direction, as explained with respectto FIG. 8.

Thus, where N represents the total number of terminal stations, or 4 inthe architecture shown in FIG. 3, N−2 output ports of an outputfrequency router associated with a multi-add/drop filter p are coupledto N−2 input ports of an input frequency router associated with amulti-add/drop filter p+1.

FIG. 4 illustrates transmissions from the first terminal station of theoptical ring network shown in FIG. 3 according to an embodiment of thepresent invention. The multiple transmitter 120 associated with thefirst station transmits λ₁, λ₂ and λ₃ into the network through anoptical fiber coupled to input port 3 of the input frequency router 530associated with the second station. As can be seen from Table I, when λ₁enters an input frequency router's input port 3, λ₁ exits from the inputfrequency router's output port 3, in this case sending information tothe second station's multiple receiver 210. This is how the firststation transmits information to the second station.

When λ₂ enters an input frequency router's input port 3, λ₂ exits fromthe input frequency router's output port 2. As shown in FIG. 4, λ₂ thentravels to input port 2 of the associated output frequency router 540.As can be seen in Table II, when λ₂ enters an output frequency router'sinput port 2, λ₂ exits from the output frequency router's output port 3.

Similarly, when λ₃ enters input port 3 of input frequency router 530, λ₃exits from the input frequency router's output port 1. As shown in FIG.4, λ₃ then travels to input port 1 of the associated output frequencyrouter 540. As can be seen in Table II, when λ₃ enters an outputfrequency router's input port 1, λ₃ exits from the output frequencyrouter's output port 3.

Thus, when the first station transmits λ₁, λ₂ and λ₃ into themulti-add/drop filter associated with the second station, λ₁ “drops”down to the second station's multiple receiver 210, and the remainingwavelengths, namely λ₂ and λ₃, pass on to the next multi-add/dropfilter.

When λ₂ and λ₃ enter input port 2 of input frequency router 550, λ₃drops down to the third station's multiple receiver 310. This is how thefirst station transmits to the third station. λ₂ passes on to the nextmulti-add/drop filter and is dropped down to the fourth station'smultiple receiver 410. This is how the first station transmits to thefourth station. Thus, by using N−1 wavelengths, or λ₁, λ₂ and λ₃, thefirst station is able to simultaneously transmit information to eachother station.

FIG. 5 illustrates transmissions from the second terminal station of theoptical ring network. The second station's multiple transmitter 220sends λ₁, λ₂ and λ₃ into the multi-add/drop filter associated with thethird station. The input frequency router 550 drops λ₁ down to the thirdstation's multiple receiver 310, and λ₂ and λ₃ pass on to themulti-add/drop filter associated with the fourth station. The inputfrequency router 570 drops λ₃ down to the fourth station's multiplereceiver 410, and λ₂ passes on to the multi-add/drop filter associatedwith the first station, where the input frequency router 510 drops λ₂down to the first station's multiple receiver 110. In this way, thesecond station transmits to the third, fourth and first stations usingλ₁, λ₃ and λ₂, respectively.

FIG. 6 illustrates transmissions from the third terminal station of theoptical ring network. The third station's multiple transmitter 320 sendsλ₁, λ₂ and λ₃ into the multi-add/drop filter associated with the fourthstation. The input frequency router 570 drops λ₁ down to the fourthstation's multiple receiver 410, and λ₂ and λ₃ pass on to themulti-add/drop filter associated with the first station. The inputfrequency router 710 drops λ₃ down to the first station's multiplereceiver 110, and λ₂ passes on to the multi-add/drop filter associatedwith the second station, where the input frequency router 530 drops λ₂down to the second station's multiple receiver 210. In this way, thethird station transmits to the fourth, first and second stations usingλ₁, λ₃ and λ₂, respectively.

FIG. 7 illustrates transmissions from the fourth terminal station of theoptical ring network. The fourth station's multiple transmitter 420sends λ₁, λ₂ and λ₃ into the multi-add/drop filter associated with thefirst station. The input frequency router 510 drops λ₁ down to the firststation's multiple receiver 110, and λ₂ and λ₃ pass on to themulti-add/drop filter associated with the second station. The inputfrequency router 530 drops λ₃ down to the second station's multiplereceiver 310, and λ₂ passes on to the multi-add/drop filter associatedwith the third station, where the input frequency router 550 drops λ₂down to the third station's multiple receiver 310. In this way, thefourth station transmits to the first, second and third stations usingλ₁, λ₃ and λ₂, respectively.

In addition to being able to simultaneously transmit to each otherstation using different wavelengths, each terminal station is able tosimultaneously receive information from each other terminal stationusing different wavelengths. For example, as explained with respect toFIG. 4, the second station receives information from the first stationusing λ₁. As explained with respect to FIG. 6, the second stationreceives information from the third station using λ₂. Finally, asexplained with respect to FIG. 7, the second station receivesinformation from the third station using λ₃.

FIG. 8 shows how the architecture of FIG. 3 may be used in abi-directional optical ring network according to another embodiment ofthe present invention. The solid arrows represent transmission mediathat have been added to the connections shown in FIG. 3. An additionaltransmission medium is used to couple each terminal station p with amulti-add/drop filter p−1, wherein the multi-add/drop filter p−1 is themulti-add/drop filter neighboring multi-add/drop filter p in the“second” direction, or from right to left in FIG. 8. The additionaltransmission medium is used to transmit information in this seconddirection.

Wavelengths may be selected such that no two of the N−1 wavelengths,such as λ₁, λ₂ and λ₃, on a given transmission medium are associatedwith communications between terminal stations in the same direction.Moreover, all of the N−1 wavelengths, such as λ₁, λ₂ and λ₃, on a giventransmission medium may be associated with a communication betweenterminal stations in either the first or second directions.

By way of example, the transmission of information from the secondstation in the second direction will now be described. The dashed arrowsshown in FIG. 8 are the same connections as were described with respectto FIG. 3, but are duplicated here for ease of explanation. The secondstation's multiple transmitter 220 transmits λ₁, λ₂ and λ₃ into outputport 1 of the output frequency router 520 associated with the firststation. The same multiple transmitter that transmits information in thefirst direction may be used, or a second multiple transmitter may beused instead, if desired.

The output frequency router 520 associated with the first station dropsλ₂ down to the first station's multiple receiver 110. The same multiplereceiver that receives information from the first direction may be used,or a second multiple receiver may be used instead, if desired. In eithercase, λ₁ and λ₃ pass on to the multi-add/drop filter associated with thefourth station, where λ₃ drops down to the fourth station's multiplereceiver 410, and λ₁ continues on to the third station's multiplereceiver 310.

In this way, if an optical fiber breaks such that a terminal station canno longer transmit to one or more remaining terminal stations in thefirst direction, that terminal station can still communicate with theremaining terminal stations in the second direction. Moreover, thecapability of the network can be doubled when there is no failure bysending information in both directions around the ring.

Finally, note that in the bi-directional network every one of thewavelength λ₁, λ₂ and λ₃ on any given optical fiber is associated with acommunication between terminal stations in either the first or seconddirection. Consider, for example, the fiber coupling output port 1 ofinput frequency router 510 with input port 1 of output frequency router520. As shown in FIGS. 6 and 7, this fiber carries λ₂ and λ₃ in thefirst direction. As shown in FIG. 8, this fiber also carries λ₁ in thesecond direction.

Although various embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and within thepurview of the appended claims without departing from the spirit andintended scope of the invention. For example, although particulararchitectures were used to illustrate the present invention, it can beappreciated that other architectures may be used instead, includingother numbers of terminals, input ports and output ports and/or theselection of different ports to couple devices. Similarly, althoughparticular devices were used within the illustrated embodiments, otherdevices will also fall within the scope of the invention.

1. An optical ring network, comprising: a number of multi-add/dropfilters, each multi-add/drop filter being coupled to two othermulti-add/drop filters to form a ring; and a number of terminalstations, each terminal station being associated with a different one ofthe multi-add/drop filters, wherein each terminal station receivesinformation from its associated multi-add/drop filter through a singletransmission medium, wherein a first one of the terminal stationsreceives information from a second different one of the terminalstations via a multi-add/drop filter associated with the seconddifferent one of the terminal stations and sends information to a thirddifferent one of the terminal stations via a multi-add/drop filterassociated with the third different one of the terminal stations;wherein each terminal station is capable of communicating with eachother terminal station simultaneously using wavelength divisionmultiplexing and N−1 wavelengths, where N is an integer greater than 1.2. The optical ring network of claim 1, wherein the multi-add/dropfilters are coupled to each other using optical transmission mediacomprising optical fibers.
 3. The optical ring network of claim 1,wherein no two of the N−1 wavelengths on a given transmission medium areassociated with communications between terminal stations in the samedirection.
 4. The optical ring network of claim 1, wherein all of theN−1 wavelengths on a given transmission medium are associated with acommunication between terminal stations in either a first direction or asecond direction.
 5. The optical ring network of claim 1, wherein eachterminal station comprises: a first multiple transmitter configured totransmit information to another one of the terminal stations in a firstdirection; and a first multiple receiver configured to receiveinformation from another one of the other terminal stations in the firstdirection.
 6. The optical ring network of claim 5, wherein each of thenumber of terminal stations further comprises: a second multipletransmitter configured to transmit information to another one of theterminal stations in a second direction; and a second multiple receiverconfigured to receive information from another one of the terminalstations in the second direction.
 7. The optical ring network of claim1, wherein each of the multi-add/drop filters comprises a pair offrequency routers.
 8. A method of communicating over an optical ringnetwork having a number N of terminal stations and associatedmulti-add/drop filters, each of the multi-add/drop filters being coupledto two other multi-add/drop filters to form a ring, comprising:transmitting information from a multi-add/drop filter to its associatedterminal station via a single transmission medium, wherein a first oneof the terminal stations receives information from a second differentone of the terminal stations via a multi-add/drop filter associated withthe second different one of the terminal stations and sends informationto a third different one of the terminal stations via a multi-add/dropfilter associated with the third different one of the terminal stations;and transmitting information from each terminal station to each otherterminal station simultaneously using wavelength division multiplexingand the N−1 wavelengths, where N is an integer greater than
 1. 9. Themethod of claim 8, further comprising: receiving information at eachterminal station from each other terminal station simultaneously, in afirst direction around the ring, using wavelength division multiplexingand the N−1 wavelengths.
 10. The method of claim 9, further comprising:transmitting information from each terminal station to each otherterminal station simultaneously, in a second direction around the ringopposite the first direction, using wavelength division multiplexing andthe N−1 wavelengths.
 11. The method of claim 8, wherein each of themulti-add/drop filters comprises a pair of frequency routers.